Previous work from this laboratory performed on bone marrow and blood cells has provided new information on the orderly step-wise assembly of cellular organelles during differentiation and their reorganization and participation in the function of these highly specialized cells. The major techniques applied are light microscopy and electron microscopy with enzyme and immunocytochemical preparations. More specifically, the present objectives of this renewal application are as follows: 1. To investigate the heterogeneity of human neutrophilic (PMN) granules by performing immunocytochemistry for the fine structural localization of selective granule proteins on bone marrow and blood leukocytes at different stages of maturation. 2. To study the receptor-mediated endocytic pathway of phagocytic leukocytes with particular emphasis on the subcellular localization of lysosomal membrane glycoproteins (LAMP's), and the fate of certain ligands and their receptors after activation. 3. To continue to study bone marrow stromal cells (macrophages and fibroblasts): 1) particularly adhesion events that occur during maturation, and, in addition 2) explore the possibility that marrow fibroblasts may be contractile cells. 4. To test bone marrow stromal cells and osteoblasts in vivo for the presence of certain growth factors by utilizing immunocytochemistry for detecting the presence of the proteins and in situ hybridization for the presence of mRNA. Compositely, this work will lead to a better understanding of differentiation, function and interrelationship of bone marrow- derived cells, particularly those responsible for the inflammatory response to infection or injury (i.e., host defense).